How to put thorns on the surface of a plant?
I had the same problem, when flower pedals were floating around the scene in random directions. I played around with particle settings and found the setting somewhere in the settings panel.

But solution to the floating particles I figured out much later. You have to make sure that your particle object is sitting right on top of its origin point. (orange dot) The particle system doesn't take the mesh and put it nicely o your object. Instead it takes the origin point and slaps it on top of the object. And when your particle object is far away from the origin, at just floats around somewhere else.



So yeah. Make sure your particle object is sitting on top of its origin (orange dot) and particle rotation settings you find somewher in the particle settings.  Hope this helped. Happy blending!